Because of the demand for corrosion resistanct metal alloys, zinc coated steel materials find an increasing use in industry, especially in the automotive field. Steel surfaces are normally coated with zinc metal either by electrolytic means, such as zinc electrocoated steel (EC), or by a dipping process in molten zinc as, for example, hot dip galvanized steel (HDG).
More recently, other coatings that contain more than one metal have been produced. We refer to coatings containing zinc/nickel and zinc/iron, which are obtained by electrodeposition, and to other coatings that have zinc/aluminum that are produced by a molten metal dip process. A zinc/iron alloy can be obtained on the surface of HDG by heat treatment (annealing).
The use of such materials in industry presents more serious problems in metal forming operations, more precisely deep drawing and stamping, than cold rolled steel sheet. This fact may be attributed to the different degree of smoothness on the metal surface.
The use of special oils for metal working, especially stamping, does not solve the problem entirely. Sometimes it can be very difficult to remove such oils after metal forming and prior to painting. Because of the heavy coating normally used (2-4 gr./m.sup.2) when wax type lubricants are used for this purpose, they are similarly difficult to remove, It is well-known that zinc coated steel has as a surface which is normally rougher or has more porosity than CRS. This fact makes oil cleaning a difficult operation. Consequently, degreasing processes become more complex.
One typical characteristic of zinc electrocoated steel is the ease with which zinc dust is loosened during sheet forming operations. This is particularly observed near the edges of the sheets where a thicker zinc coating, which consequently is less adherent, is formed. This leads to early wearing out or dusting of stamping presses or other tools.
In order to avoid premature oxidation of metal sheets during long periods of storage and transport, it is a common practice to protect zinc materials with rust inhibitor oils. The composition of antioxidant oils may be altered during long periods of exposure to light and ambient conditions. Sometimes the oils become physically occluded inside the porous surface of the metal.